Four versus one doesn't sound like a fair fight at all. But for Chris Ashton, cofounder and design director at Turtle Rock Studios, evening the odds of a four-on-one fight is one of his main focuses with the upcoming online sci-fi shooter, Evolve.

Ashton and his crew at Turtle Rock are the people who created Left 4 Dead, the hit four-player online co-op shooter series developed with Valve that pitted players against procedurally-spawned zombie hordes, driven by what the studio dubbed the "AI director."

So, the team knows a thing or two about four-player co-op. What Evolve does, however, is replace the AI-driven zombies of Left 4 Dead with a player-controlled monster (called the "Goliath") that becomes increasingly powerful as time goes on, and as it consumes local wildlife. As long as it stays alive, it progressively becomes more of a threat against the other four people playing as hunters, who play as four distinct classes. As a battle escalates, the designations of "hunted" and "hunter" become less defined. The monster can win if it takes out all the hunters or by eliminating specific targets on the map; hunters win if they can kill the monster.

Ashton says development of Evolve was built upon lessons learned from Left 4 Dead, such as keeping a focus on replayability and making teamwork a requirement. But there are other notable keys to effective online multiplayer design that are being implemented in Evolve.

Make co-op easy

"We focused on making co-op easy. That's something that we also did in Left 4 Dead, but since Evolve is a deeper game, it was a challenge that kept coming up. That's where classes came from," Ashton says.

There are four hunter classes in Evolve: assault, medic, support and trapper. In previous iterations of the game, players were able to pick and choose gear before entering a game - a common feature in many online shooters. But Ashton said that led to a lot of confusion among players, and any confusion gets in the way of cooperation. In order to reach the design goal of making co-op easy, the designers took away options from players by creating specific classes. That accessibility is necessary if players are joining random games with random people with various skill levels.

"The confusion went away - you're the medic now, and you know what you're supposed to do, and I know what you're supposed to, so we both know what to do out of the gate," says Ashton.

Let players make mistakes

Right alongside the goal of making co-op easy in Evolve is the idea that players should be allowed to make mistakes, and at the same time be given the opportunity to avoid making those mistakes. When a team comes together and minimizes the amount of mistakes they collectively make to pull off a win, that's a rewarding feeling.

"If you want to have a difference between a bad team and a good team - which is what I think you want in co-op - then you have to allow for people to make mistakes," says Ashton.

An example of this is how in Evolve, a trapper might deploy a mobile arena (a large dome meant to trap the Goliath inside for close combat with hunters) in the wrong area, trapping hunters inside while the monster runs free. Instead of letting hunters immediately escape that dome to continue pursuing the monster, the game is designed so hunters are trapped inside until the mobile arena expires or the trapper brings it down, giving the monster-player more time to evolve, and grow stronger.

The person playing as a trapper messed up, and poor teamwork is punished, while the person playing the monster gains an advantage. This ramps up the importance of avoiding mistakes, of which there are plenty of opportunities to make.

Pay attention to data, but also listen to players

Ashton and his team balance the game through playtesting, monitoring two variables: Player perception (interviews of players on what they thought or felt about the experience), and telemetry (large amounts of data such as where players are on a map, how much health they have, what weapons they're using, etc., at any point in time).

"A key thing for us [to keep in mind] is that the player perception is an equal to the telemetry system," Ashton says. "If everyone says 'This gun is overpowered,' yet in the telemetry system, according to the numbers, it's not, I'm not just going to close the book and say, 'hey, it's not overpowered, according to the telemetry.' Perception is reality for a player.

"For us, we have to dig deeper, and find out why players feel that gun is overpowered, and we can use telemetry to dig in a little bit deeper to solve that problem. It's always those two things on which we balance the game - both on player perception and data."

Different player roles, all equally important

For Ashton and the team working on Evolve each player on the hunter side is equally important. A design goal was to make it so that each hunter didn't represent just 25 percent of a team of four, but that each role was absolutely vital.

"On the hunter side, coming away from it, the big goal is that we wanted to make sure that each role was critical to the survival and success of the team - like a table with four legs; take one out, and the table falls over," says Ashton.

Making sure the monster was balanced against that "table" meant that no matter which of its three stages of evolution the Goliath is in, a good team of hunters would be able to pull off a victory. The early game had to be just as balanced as the late game.

"It was critical to make sure the outcome of a game is up to the players, and what they do," says Ashton. "It's been really fun. It's just a process, and it takes time, playtesting and observing. I'm sure when the game comes out, people will be doing things that we've never seen. And we'll be there looking at the telemetry, listening to player feedback, and making adjustments at the very least."

Evolve is published by 2K, and is slated for a fall release on Xbox One, PlayStation 4 and PC.